Cement clinker ( Portland cement clinker , short: clinker) is the burnt component of the cement that is responsible for hardening when water is added. The name clinker comes from the similarity to high-fired ( sintered ) bricks ( clinker ). However, cement clinker should not be confused with the brick product .
Clay minerals and lime are mainly used as starting products, mostly in the form of clay and calcareous rocks such as limestone or marl . The raw materials are ground and then heated at around 1,450 ° C until they partially fuse together ( sintering ). The now spherical material is cooled and ground again to obtain cement . Quartz sand and materials containing iron oxide can be added before firing to improve sintering. In order to obtain cement types with certain properties, blast furnace slag , fly ash and gypsum can also be added, for example .
The four phases of clinker essentially arise during sintering:
- Tricalcium Silicate (Alit) C3S
- Dicalcium silicate (belite) C2S
- Tricalcium aluminate C3A
- Tetracalcium aluminate ferrite C4AF
The clinker mainly consists of the following phases :
|Clinker phase||chemical formula||Short formula|
|Tetracalcium aluminate ferrite||Brownmillerite|
|Tricalcium aluminate||Calcium aluminate|
|free CaO||- Free lime|
The most important phase for hardening is the tricalcium silicate, which has the highest initial hardness.
From a chemical point of view, Portland cement clinker consists of the following main components:
|component||Average share [% by mass]|
The main component alite is formed in the presence of a melting phase from around 1250 ° C. In order to achieve a sufficiently complete reaction (residual free lime content below 1.5%), temperatures of around 1450 ° C. are necessary for a period of 5 minutes; there should prevail a predominantly oxidizing atmosphere in the combustion unit and the precursors belite and free lime are homogeneously.